ASP.NET Upload file failing

[Mark]

I am using a Telerik Upload control. I have increased the maxRequestLength and maxAllowedContentLength appropriately. I have set the executionTimeout to 600 seconds and temporarily disabled anti-virus. Small files (< 4MB) upload fine. Too large files (>32MB) are rejected by the server. But, files in between result in the IEnumerable<HttpPostedFileBase> argument being null. I can see the server creating the temporary files in the Temporary ASP.NET folder. The .post file is there and seems to have an appropriate planned size. But, when the timeout happens, it has not been completed and my code is called with a null. It seems odd coincidental that the break point is 4MB, but I cannot find any other configuration parameter controlling this.